In recent years, there has been growing demand for making a thick optical element by injection molding. An increase in thickness of a plastic optical element leads to an increase in stress caused by the difference in shrinkage on curing between a surface plastic layer that cures first during molding and an inner plastic portion that cures subsequently. Disadvantageously, this causes a vacuum bubble (void) in the optical element or allows inner stress to remain therein. Furthermore, since the time required to cool such a thick optical element in a mold drastically becomes long due to the increase in thickness, a molding cycle markedly increases.
To overcome the above-described disadvantages, PTL 1 discloses a method of making a plastic optical element including a plastic core lens internally disposed and plastic coatings which cover front and rear optical surfaces of the core lens such that the core lens is integrated with the plastic coatings.
In a typical optical element made by injection molding, shear stress occurs upon injection to induce unidirectional alignment of a polymer chain in a material to be molded, thus causing optical anisotropy. This optical anisotropy caused by the polymer chain alignment is called birefringence, which reduces the light gathering power of a lens.
The method disclosed in PTL 1 has a disadvantage in that birefringence in the core lens is combined with that in the plastic coating on each surface of the core lens such that the resultant birefringence is worse than birefringence in an optical element produced by typical injection molding.